1. Field of the Invention
The present invention relates to systems and methods of stationkeeping satellite orbits, and in particular to a method and system for controlling eccentricity in inclined geosynchronous orbits.
2. Description of the Related Art
Over the past several decades, there has been a dramatic increase in the number of satellites in orbit. These satellites are maintained in different orbits, which are selected to allow the satellite to perform its intended mission.
Geostationary satellite orbits are highly valued, because they allow the satellite to remain in a fixed apparent location from a reference on the earth's surface. When satellites are placed in such orbits, the ground stations can direct the transceiving antenna at a fixed direction.
FIG. 1A is a diagram of a satellite 104 disposed in an orbit 106 around the earth 102. The satellite 104 is maintained in the desired orbit 106 by performing stationkeeping satellite maneuvers. These maneuvers can be performed autonomously by the satellite 104 itself using a satellite control system 110 having one or more thrusters 118 and one or more satellite processors 112 implementing one or more instructions for commanding the thrusters 118 to perform stationkeeping maneuvers. Alternatively or in addition to the autonomous stationkeeping technique, a command facility 114 having a command facility processor 116 communicatively coupled to a transmitter 120 can generate and transmit stationkeeping commands to the satellite 104. Although illustrated as a ground-based command facility, the stationkeeping commands may be transmitted from another spacecraft as well.
FIG. 1B is a diagram showing a first satellite 104A in a geostationary orbit 106A. Due to satellite-to-satellite communication interference issues, satellites in such geostationary orbits 106A are assigned to geostationary "slots" that are 0.2 degrees wide (.+-.0.1 degree about the nominal longitude). These satellites must remain within the assigned slot.
FIG. 1B also depicts a second satellite 104B in a geosynchronous orbit 106B. Geosynchronous orbits, which are often used for communications to mobile customers (such as with "GEOMOBILE" satellites) are similar to those of geostationary orbits, except, they have a non-zero inclination typically in the range of three to seven degrees.
FIG. 2 is a diagram depicting the ground track 202 of a typical geosynchronous orbit 106B. The center 204 of the "figure-8" depicted by the ground track 202 is at the equatorial plane of the earth 102. The satellite 104B passes through the ground track center 204 twice each day, once at the ascending node 208 and once at the descending node 210. The motion is more complex in practice, due to orbit eccentricity, drift and perturbing forces. Despite the satellite 104B motion, interference is still a problem, and the satellite 104B is therefore still constrained to the .+-.0.1 degree slot near the equatorial plane. The geosynchronous satellite 104B cannot stay inside the .+-.0.1 degree slot all of the time (the width of the ground track itself exceeds the .+-.0.1 degree window by itself when the orbital inclination exceeds about 4.8 degrees).
This problem is especially critical for current and future generation spacecraft. Such spacecraft often have large solar arrays and solar collectors, and therefore receive a strong solar force. This solar force produces a large steady state eccentricity when a single burn sun-synchronous perigee stationkeeping strategy is used. This eccentricity is difficult to control efficiently, even when a sun facing perigee stationkeeping strategy (which compresses eccentricity using drift control maneuvers) is used to conserve fuel. In some satellites, the east/west longitude excursion due to eccentricity can take up more than half the width of the slot. Other factors also consume slot 108 width, including drift over the maneuver cycle, maneuver execution error, bipropellant momentum dumping disturbances, orbit determination error, and orbit propagation error. A discussion of these contributors is presented in "The Operation and Service of Koreasat-1 in Inclined Orbit," AIAA paper 98-1352, which reference is incorporated by reference herein. Hence, closer control of the excursion due to eccentricity is required.
There are many possible solutions to this problem. One is to introduce a maneuver scheduler, allowing drift maneuvers to be performed daily. This solution would require more complicated satellite processing. Further, this solution would be difficult to implement for satellites already on-orbit, and would only be helpful at "high-drift" longitudes.
Another possible solution is to use a "two-maneuver" satellite maneuver scheme which performs burns in both tangential directions to maintain tighter control over the eccentricity. However, this solution requires more propellant, additional thrusters, and raises issues of additional plume impingement.
Another potential solution is to implement an axial firing mode in which radial .DELTA.V is used to control eccentricity. This solution uses roughly twice the fuel of a tangential .DELTA.V scheme, and would require modified maneuver planning algorithms, and changes to the attitude control system (ACS).
Another potential solution would be to implement a shorter orbital stationkeeping maneuver interval. This would not provide any propellant savings, but would reduce longitudinal drift between maneuvers. However, this is a manual equivalent to the first proposed solution, and the shortened stationkeeping maneuver interval would increase support operations costs.
As is apparent from the foregoing, there is a need for a stationkeeping method for satellites in geosynchronous orbits that provides the necessary longitudinal control near the equatorial plane, without requiring the operational, hardware, or software costs outlined above. The present invention satisfies that need.